There is a requirement to measure various parameters of a textile yarn during and subsequent to the processing of the yarn, to determine the efficiency and effectiveness of such processing and the quality of the processed yarn. To this end, there are many sensing instruments available, each adapted to measure a specific parameter. A typical yarn process on POY or DTY may require the measurement of interlace, oil level and tension and the determination of the presence of broken filaments. To perform all of these measuring or sensing functions requires the use of at least three sensing instruments. Whilst some of the actual operative sensing elements may be non-contact, e.g. optical sensors, each sensing instrument requires two or more yarn contacting sensing elements and/or yarn guides to locate the running yarn in the sensor relative to the operative sensing elements. The result of adding three sensing instruments to a typical process threadline can add over 45.degree. of sensing element/guide wrap, thereby undesirably increasing by a considerable amount the tension in the yarn downstream of those instruments. Hence the quality of the processed yarn is often compromised by the act of monitoring its quality. For this reason, and having regard to the high process speed with some processes, it is desirable or even essential to minimise the total sensing element/guide contact with the yarn within the complete process threadline.